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Stanford CoreNLP provides a set of natural language analysis tools which can take raw English language text input and give the base forms of words, their parts of speech, whether they are names of companies, people, etc., normalize dates, times, and numeric quantities, mark up the structure of sentences in terms of phrases and word dependencies, and indicate which noun phrases refer to the same entities. It provides the foundational building blocks for higher level text understanding applications.

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package edu.stanford.nlp.trees;

import java.util.regex.Pattern;

import edu.stanford.nlp.trees.tregex.TregexPattern;
import edu.stanford.nlp.trees.tregex.tsurgeon.Tsurgeon;
import edu.stanford.nlp.trees.tregex.tsurgeon.TsurgeonPattern;

/**
 * Transforms an English structure parse tree in order to get the dependencies right:  

 *  -- put a ROOT node  

 *  -- remove NONE nodes  

 *  -- retain only NP-TMP, NP-ADV, UCP-TMP tags  

 * The UCP- tags will later be turned into NP- anyway 

 *
 * (Note [cdm]: A lot of this overlaps other existing functionality in trees.
 * Could aim to unify it.)
 *
 * @author mcdm
 */
public class DependencyTreeTransformer implements TreeTransformer {


  private static final Pattern TmpPattern = Pattern.compile("(NP|UCP).*-TMP.*");
  private static final Pattern AdvPattern = Pattern.compile("(NP|UCP).*-ADV.*");
  protected final TreebankLanguagePack tlp;

  public DependencyTreeTransformer() {
    tlp = new PennTreebankLanguagePack();
  }

  @Override
  public Tree transformTree(Tree t) {
    //deal with empty root
    t.setValue(cleanUpRoot(t.value()));
    //strips tags
    stripTag(t);

    // strip empty nodes
    return stripEmptyNode(t);
  }

  protected static String cleanUpRoot(String label) {
    if (label == null || label.equals("TOP")) {
      return "ROOT";
      // String constants are always interned
    } else {
      return label;
    }
  }

  // only leaves NP-TMP and NP-ADV
  protected String cleanUpLabel(String label) {
    if (label == null) {
      return "";  // This shouldn't really happen, but can happen if there are unlabeled nodes further down a tree, as apparently happens in at least the 20100730 era American National Corpus
    }
    boolean nptemp = TmpPattern.matcher(label).matches();
    boolean npadv = AdvPattern.matcher(label).matches();
    label = tlp.basicCategory(label);
    if (nptemp) {
      label = label + "-TMP";
    } else if (npadv) {
      label = label + "-ADV";
    }
    return label;
  }

  protected void stripTag(Tree t) {
    if ( ! t.isLeaf()) {
      String label = cleanUpLabel(t.value());
      t.setValue(label);
      for (Tree child : t.getChildrenAsList()) {
        stripTag(child);
      }
    }
  }

  private static final TregexPattern matchPattern =
    TregexPattern.safeCompile("-NONE-=none", true);

  private static final TsurgeonPattern operation =
    Tsurgeon.parseOperation("prune none");

  protected static Tree stripEmptyNode(Tree t) {
    return Tsurgeon.processPattern(matchPattern, operation, t);
  }

}